School of Materials, University of Manchester, Oxford Rd., Manchester M13 9PL, UK.
Langmuir. 2012 Mar 27;28(12):5331-8. doi: 10.1021/la204440w. Epub 2012 Mar 15.
During the drying of drops of nanoparticle suspensions, segregation can occur by internal fluid flows toward the contact line, if the contact line is pinned. This leads to a characteristic ring deposit or coffee stain. On solid substrates coffee staining can be eliminated through the use of solvent mixtures that promote Marangoni flows to oppose these drying-induced flows. Here it is shown that a suspension, optimized to eliminate the formation of coffee stains on a range of solid surfaces, shows coffee staining on a number of porous surfaces. This behavior is shown to be consistent with a mechanism of fluid removal through capillary flow (draining) of the solvent into the porous substrate, combined with filtration of the particles by the small pore size, in addition to the flow from solvent evaporation. The extent of capillary driven coffee staining is a function of substrate pore size: if the pore size is small, capillary flow is slow, reducing the observed coffee staining. However, if the pore size is too large, the nanoparticles are absorbed into the material along with the draining solute and no deposition of particles is observed.
在纳米颗粒悬浮液液滴干燥过程中，如果接触线被固定，内部流体可能会流向接触线，从而发生分相。这会导致出现特征性的环形沉积物或咖啡环。在固体基底上，可以通过使用促进 Marangoni 流以对抗这些干燥诱导流的溶剂混合物来消除咖啡环。本文表明，优化后的悬浮液可消除在一系列固体表面上形成咖啡环，但在一些多孔表面上仍会出现咖啡环。这种行为与通过溶剂通过毛细作用（排液）进入多孔基底以及通过小孔径过滤颗粒的机制一致，此外还存在溶剂蒸发引起的流动。毛细驱动咖啡环的程度是基底孔径的函数：如果孔径小，毛细流动缓慢，会减少观察到的咖啡环。但是，如果孔径太大，纳米颗粒会与排液的溶质一起被吸收到材料中，并且不会观察到颗粒沉积。
